build_core: silence some stderr

git-svn-id: https://rebeccablackos.svn.sourceforge.net/svnroot/rebeccablackos@2941 56e3d1a8-0179-4092-b3e2-9fbbcda0c0a9
weston-video-convert-helper-264: only run when there is a capture file.

git-svn-id: https://rebeccablackos.svn.sourceforge.net/svnroot/rebeccablackos@2942 56e3d1a8-0179-4092-b3e2-9fbbcda0c0a9